Tobias sent a chain letter to his friends, asking them to forward the letter to more friends. The number of people who receive the email increases by a factor of $4$ every $9.1$ weeks, and can be modeled by a function, $P$, which depends on the amount of time, $t$ (in weeks). Tobias initially sent the chain letter to $37$ friends. Write a function that models the number of people who receive the email $t$ weeks since Tobias initially sent the chain letter. $P(t) = $
Explanation: The strategy We can model the situation with an exponential function of the general form A ⋅ B f ( t ) A\cdot B\^{ f(t)}, where $A$ is the initial quantity, $B$ is a factor by which the quantity is multiplied over constant time intervals, and $f(t)$ is an expression in terms of $t$ that determines those time intervals. Let's use the given information to determine $A$, $B$, and $f(t)$. Understanding what's given We are given that the initial number of people who receive the email is $37$, and the number of people who receive the email increases by a factor of $4$ in $9.1$ weeks. This means that the initial quantity is $A=37$ and the factor is $B=4$. We need to find $f(t)$ based on the fact that the quantity is multiplied by $4$ every $9.1$ weeks. Finding the expression in the exponent We know that the number of people who receive the email is multiplied by $4$ every $9.1$ weeks. This means that each time $t$ increases by $9.1$, $f(t)$ increases by $1$. Therefore, $f(t)$ is a linear function whose slope is $\dfrac{1}{9.1}$. When Tobias initially sent the e-mail, the number of people who have received the e-mail has not begun to increase yet. So $P(0) = 37$, which means that $f(0)=0$. [Why?] Therefore, $f(t)$ must be $\dfrac{t}{9.1}$. Summary We found that the following function models the number of people who receive the email $t$ weeks since Tobias initially sent the chain letter. P ( t ) = 37 ⋅ 4 t 9.1 P(t)=37\cdot 4\^{ \frac{t}{9.1}}